1. Field of the Invention
The present invention relates to non-volatile semiconductor storage devices, in particular to a non-volatile semiconductor storage device operating under a method of reading out data from a cell of the virtual ground type, and a method of reading out data.
2. Description of the Related Art
Recently, with the spread of portable equipment, memories are shifting to become of greater capacity and lower voltage. Furthermore, with the speed-up in the operation of the CPU, there is a stronger demand to speed-up the operation of memory. The virtual ground type is known as a way to achieve a large capacity memory. The virtual ground type increases the density of the cells by reducing the number of contacts of the cell and thus is suitable for a large capacity memory. However, the method of reading out data from the cell becomes more complex and thus is not suitable for a low voltage and high-speed memory.
Conventionally, in regards to reading out data from cells of the virtual ground type, a method (hereinafter also referred to as the first conventional method) of pre-charging all of the bit lines to cancel out the leak current from the adjacent cells and then detecting the level or the current on the side of the drain; and a method (hereinafter also referred to as the second conventional method) of detecting the level on the side of the source where the influence of the leak current of the cells is smaller, are used. The second conventional method is disclosed in, for example, Japanese Laid-Open Patent Application 3-176895.
In the first conventional method, since the bit lines are pre-charged, the power consumption is increased.
Furthermore, in the second conventional method, because the operation of reading out data from the side of the source is carried out between the ground potential and the positive potential, a problem arises in that it becomes difficult to maintain a sufficient margin for reading. This problem becomes more significant as the operating voltage decreases. Moreover, considering the fact that leak current from the adjacent cells still exists even when reading from the side of the source, there is a limit in extending the margin for reading.
Accordingly, it is a general object of the present invention to provide a non-volatile semiconductor storage device and a method of reading out data that solves the problems of the prior art and ensures sufficient reading margin.
In order to achieve the above object, the non-volatile storage device of the present invention is a non-volatile semiconductor storage device provided with a boost circuit for setting, for at least a certain period of time, a source line selectively connected to a memory cell to a negative potential, when reading out data from the memory cell.
As the potential of the source line is set to a negative potential, when the memory cell is a write cell, the source line remains at a negative potential even if the data is read out from the memory cell, whereas when the memory cell is an erase cell, the current flows into the source line and the potential of the source line increases beyond the ground level to a positive potential. Thus, the reading margin is the difference between such negative potential and the positive potential and an extended reading margin can be obtained. Even at the time of low-voltage operation, the reading margin can reliably sense the change in the potential of the source line because the negative voltage is set as the reference voltage.